The objective of the proposed research is to discover the basic cellular mechanism that permit the retinal pigment epithelium (RPE) to phagocytize and digest the terminal disks of the rod outer segments (ROS). Utilizing an organ cultured RPE explant as a model system for phagocytosis, we will test young and senile, macular and peripheral human RPE for their relative ability to phagocytize inert particles. The effect of lipofuscin content of RPE on phagocytic ability will be evaluated. The biological life history of melanin in RPE will be evaluated using ultrastructural enzyme cytochemical techniques. The effects of ultrastructural abnormalities of Bruch's membrane on phagocytic performance will be assessed. Bovine RPE explants will be used to test specific steps in acute phagocytosis, particularly possible shifts in synthetic activities. The digestive process will be studied by feeding RPE radiolabeled ROS and determining the rate and pattern of breakdown of macromolecules by RPE lysosomal enzymes.